Anti-AChR T helper (Th, CD4+) cells and antibodies (Ab) in Myasthenia Gravis (MG) and in murine; Experimental MG (EMG) recognize many epitopes, but evidence suggests that only anti-AChR subpopulations of particular epitope specificity cause myasthenic symptoms. Given the preferential cooperation between T and B cells of certain epitope specificities, it is likely that only certain anti-AChR Th/B pairs will have strong pathogenic potential. The overall goal of the proposed project is to investigate the relative pathogenic potentials of human anti-AChR CD4+ cells of different epitope specificity, and their ability to support synthesis of Abs of highly pathogenic potential, and to identify the epitope recognized by pathogenic Abs. The specific aims will be: 1) To identify the frequency in the blood of MG patients of T cells recognizing individual muscle AChR subunits and epitopes, by an immunospot assay, using unselected blood CD4+ cells and synthetic peptides screening the AChR subunit sequences. 2) To determine the pathogenic potential of anti-AChR CD4+ cells specific for defined epitope sequences, by engrafting immunosuppressed RAG-2 knock out or SCID mice with defined mixtures of CD4+ depleted PBL from MO patients, plus anti-AChR CD4+ lines of defined epitope specificity. 3) To determine the TCR V region usage-of the epitope-specific human anti-AChR CD4+ line by a PCR assay, using primers specific for the known TCR Valpha and Vbeta families. 4) To investigate the V region sequence and the pathogenic potential of anti-AChR Abs in MG patients, using monoclonal B cell lines derived from B cells present in thymus of MG patients.